The present invention relates to a printer apparatus which includes a mechanism for adjusting the gap between the surface of a print sheet and the print head.
Generally, in wire-dot printers, the print head is mounted on the carriage which moves in the axial direction of the platen. Printing is performed by pressing an ink ribbon against the paper by an impact from the print wires that extend from the print head.
In general, a printer does not always use the same thickness of paper, since different types of printing require the use of different thicknesses of paper. For example, when a multiple-part paper stock (a set of sheets arranged in layer-like fashion, with carbon paper placed between adjacent sheets, to obtain multiple printed copies) is used, it is necessary to adjust appropriately the gap between the surface of a print sheet and the print head, in accordance with the thickness of the paper placed on the platen.
A printer developed to accommodate differences in paper thickness is disclosed in U.S. Pat. No. 4,652,153, granted to Kotsuzumi et al., and in U.S. Pat. No. 4,676,675 granted to Suzuki et al. which adjust the gap automatically, according to the thickness of the paper placed on the platen. For printer apparatuses provided with this type of automatic gap-adjusting mechanism, the shaft supporting the carriage is mounted such that it is movable in the direction of the platen, and the size of the gap is changed by moving the shaft using a stepping motor. When paper is wrapped around the platen, the print head moves toward the platen, contacts the platen and moves back a specified distance, thereby obtaining a gap corresponding to the thickness of the paper. The stepping motor steps out when the print head, moving forward, contacts the platen. More specifically, the rotor of the stepping motor is caused to stop in spite of the changeover of the exciting phases.
In the case of printer apparatuses having an automatic gap adjusting mechanism, the same level of torque is produced by the stepping motor when the print head is moved toward the platen (forward movement) or moved away from the platen (backward movement). This phenomenon poses a number of problems, regardless of whether the torque is high or low. The problems in question will be described in detail in the following.
Assume that the print head is made to move forward perpendicularly to the platen by means of a high level of torque supplied by the stepping motor. In this case, even when the print head contacts the platen, the motor does not step out instantly, therefore the print head presses against part of the surface of the platen considerably before the motor steps out. This makes it impossible to obtain a precise gap.
Suppose that the torque of the stepping motor is set at a low level. In this case, when the print head moves and contacts the platen, the stepping motor steps out. The exciting phases of the stepping motor at this time must remain unchanged. However, since the surface of the platen is made of rubber, it exerts a reactive force when it receives an impact given from the print head. This reactive force causes improper exciting phases. In addition, when the print head moves backward, owing to the synergetic effect of the reactive force of the platen and the torque of the stepping motor, the exciting phases are further shifted incorrectly.
A four-phase stepping motor can be driven by two-phase excitation as is shown in FIG. 3. When the magnet rotor is positioned as shown in FIG. 1, signals S3, S4, S5 and S6 are applied to transistors Tr1 to Tr4, turning on these transistors. Hence, coils L1 and L4 are excited, causing a current to flow as is shown in FIG. 1, coils L1 and L4 function as electromagnets. Therefore, S poles occur in coils L1 and L4 at their sides facing the magnet rotor, in accordance with the right-hand thumb law. Consequently, the N pole of the rotor moves to the mid-point between coils L1 and L4. Then, signals S3, S4, S5 and S6 are applied to transistors Tr1 to Tr4, turning on transistors Tr1 and Tr2. As a result, coils L1 and L2 are excited, and S poles occur in coils L1 and L2 at their sides facing the magnet rotor. Consequently, the N pole of the magnet rotor moves to the mid-point between coils L1 and L2. Similarly, when transistors Tr2 and Tr3 are turned on, coils L2 and L3 are excited, and the N pole of the magnet rotor is positioned at the mid-point between coils L2 and L3; and when transistors Tr3 and Tr4 are turned on, coils L3 and L4 are excited, and the N pole of the magnet rotor moves to the mid-point between coils L3 and L4.
For a stepping motor which is driven by two-phase excitation as mentioned above, a central processing unit (CPU), for example, which supplies signals to change the exciting phase, checks the output signal of a photo-sensor, for example, which has detected the rotation of the stepping motor before the change of the exciting phase. After the rotation of the stepping motor becomes steady, the CPU changes the exciting phase to the next one.
Suppose that the print head contacts the platen when the exciting phase is changed from CD to DA during forward movement of the print head. The torque of the stepping motor is low at this time. The stepping motor steps out and the exciting phase is held at DA. Then, the exciting phase is shifted from, DA to CD, whereby the print head moves backward. Further, reactive force is applied from the platen. Both this reactive force and the torque of the head moving backward sometimes cause the magnet rotor to make nearly one rotation, resulting in an erroneous shift of about four phases.
Suppose that the print head contacts the platen when the exciting phase is changed from CD to DA during forward movement of the print head. The torque of the stepping motor is high in this case. Due to the high torque, the stepping motor does not step out but continues to rotate. The CPU changes the exciting phase from DA to AB. Therefore, the print head contacts the platen with an excessively great force, whereby the platen is depressed. This depression makes it impossible to obtain a precise gap. This is the disadvantage of printer apparatuses having the conventional automatic gap adjusting mechanism.